Following fractionation or distillation of crude petroleum oil, a straight-run naphtha fraction, boiling in the 70° C. to 190° C. range, is obtained. This fraction may be catalytically converted to an aromatic reformate.
On conversion to reformate, the aromatics content is considerably increased and the resulting hydrocarbon mixture becomes highly desirable as a source of valuable chemical intermediates and as a component for gasoline.
Heart-cut reformate generally contains aromatic hydrocarbons having 8 carbon atoms such as ethylbenzene and xylenes. Other components may be present such as their hydrogenated homologues such as naphthenes.
Within the xylenes, para-xylene is the most useful commodity and isomerisation or transalkylation processes have been developed to increase the proportion of para-xylene. However, isomerisation or transalkylation processes can also produce undesired side-products such as compounds having of from 1 to 5 carbon atoms, toluene, compounds having 9 or more carbon atoms and benzene.
Many catalysts have been made and proposed for various reactions involving aromatics, but for some reactions, such as ethylbenzene isomerisation or transalkylation processes, there is commonly a trade-off between providing the desired products and known side reactions. One common side reaction of ethylbenzene hydroisomerisation is the formation of compounds having of from 1 to 5 carbon atoms, which is disadvantageous from an environmental and economic point of view.
U.S. Pat. No. 4,939,110 discloses a catalyst comprising an inorganic oxide binder, a pentasil zeolite, a Group VIII metal and lead for use in the conversion of aromatic hydrocarbons.
U.S. Pat. No. 4,762,957 discloses a process for the isomerisation of alkylaromatics using a catalyst with an alumina matrix, a magnesium-containing zeolite, and a Group VIII metal component.
Whilst reasonable results are presented in these documents, the inclusion of magnesium or iron adds complexity and expense to the catalyst preparation.
WO9745198 discloses a zeolite bound zeolite catalyst for use in hydrocarbon conversion comprising first crystals of a first zeolite, and a binder comprising second crystals of a second zeolite which has a structure type that is different from the structure type of the first zeolite. The first and second zeolites provide a bifunctional catalyst having the capability of performing two or more functions. However, the production of such a zeolite bound zeolite catalyst requires additional complexity and manufacturing steps, so that zeolite bound zeolite catalysts have not apparently been scaled up commercially.
U.S. Pat. No. 3,856,872 describes a xylene isomerization process in which the conventional platinum on silica-alumina is replaced by a zeolite catalyst of the ZSM-5 type or a zeolite ZSM-12 catalyst or zeolite ZSM-21 catalyst. The zeolite can be incorporated in an inert, and therefore non-acidic, alumina matrix. U.S. Pat. No. 3,856,872 contains no teaching on kind or amount of zeolite ZSM-21.
The article “Dealuminated zeolite-based composite catalysts for reforming of an industrial naphthene-rich feedstock” deals with reforming catalysts for naphthenic feedstocks. The preferred ZSM-12 zeolite has a Si/Al ratio of 54. It is taught that ZSM-12 having higher silica to alumina ratios should not be used for such catalysts as this produces large amounts of CH4 and less aromatics.